To reduce harmful substance contained in exhaust gases discharged from engines of automobiles, etc. for the protection of regional and global environment, exhaust-gas-cleaning catalytic converters and particulate-matter-capturing ceramic honeycomb filters comprising ceramic honeycomb structures are used.
As shown in FIG. 2, a conventional ceramic honeycomb structure 20 comprises large numbers of flow paths 14 defined by perpendicularly crossing cell walls 13 and an outer peripheral wall 21, usually having a substantially circular or elliptical vertical cross section in a flow path direction. The outer peripheral wall 21 of the ceramic honeycomb structure 20 is held by a grip member (not shown) formed by a metal mesh, a ceramic mat, etc. in a metal container (not shown).
The ceramic honeycomb structure 20 is produced by the steps of (1) mixing and blending starting materials comprising ceramic materials such as cordierite powder, a molding aid, a pore-forming material, etc. with water to prepare a moldable ceramic material, (2) extruding the moldable ceramic material through a honeycomb-shaped die to produce a green ceramic honeycomb body integrally comprising an outer peripheral wall 21 and cell walls 13, and (3) drying and sintering the green body. Such steps provides a ceramic honeycomb structure 20 having predetermined shape and strength, with cell walls 13 having fine pores.
Used for filters for cleaning exhaust gases from diesel engines may be large ceramic honeycomb structures 20 of 150 mm or more in outer diameter Da and 150 mm or more in length L with cell walls 13 as thin as 0.2 mm or less, in FIG. 2. In the production of such large ceramic honeycomb structures 20 with thin cell walls, green ceramic honeycomb bodies obtained by extruding moldable ceramic materials have such insufficient strength that they are likely deformed with cell walls 13 in edge portions of their outer peripheral walls 21 crushed by their own weight. The sintering of deformed green bodies would not provide ceramic honeycomb structures 20 with predetermined strength.
To solve this problem, as shown in FIGS. 3(a) and 3(b), JP 5-269388 A discloses a honeycomb structure 10 obtained by filling a coating material comprising cordierite particles and/or ceramic fibers and colloidal oxide (colloidal silica, colloidal alumina, etc.) as main components in grooves 15 of cells 14a on an outer peripheral surface, among large numbers of cells 14 defined by cell walls 13, and drying or sintering it to form an outer peripheral wall 12 having a thickness T on a ceramic honeycomb body 11, such that the honeycomb structure 10 has an outer diameter Dd. JP 5-269388 A describes that the outer peripheral wall 12 reinforces an outer peripheral surface 11a, thereby providing the honeycomb structure 10 with excellent heat resistance and thermal shock resistance.
JP 2002-166404 A proposes a method for producing a ceramic honeycomb structure 10 having a uniform outer size, comprising applying a coating material to an outer peripheral surface 11a of ceramic honeycomb body 11, from which a peripheral portion is removed by machining, using an apparatus shown in FIG. 4(a), and drying the resultant coating to an outer peripheral wall 12. The apparatus shown in FIG. 4(a) comprises a pair of center members 42 for sandwiching center portions of both end surfaces 15a, 15b of the ceramic honeycomb body 11, and a pair of outer members 41 each surrounding the center member 42 for sandwiching outer portions of both end surfaces 15a, 15b. Each center member 42 comprises a planar support plate 42a and a shaft member 42c fixed to the support plate 42a, and each outer member 41 comprises a flat, hollow support plate 41a, a hollow shaft member 41c fixed to this support plate 41a, and a portion 41b having an outer diameter Dc larger than the outer diameter Db of the outer peripheral surface 11a of the ceramic honeycomb body 11 for abutting a scraper 43. The method described in JP 2002-166404 A comprises sandwiching both end surfaces 15a, 15b of the ceramic honeycomb bodies 11 with the support plates 41a, 42a, abutting the scraper 43 to the portion 41b, rotating the shaft members 41c, 42c filling a coating material in a gap defined by the outer peripheral surface 11a of ceramic honeycomb body 11, the support plates 41 and the scraper 43, and drying the coating material applied to the outer peripheral surface to form the outer peripheral wall 12 shown in FIG. 4(b). This reference describes that by providing each sandwiching member with a two-part structure comprising the center member 42 and the outer member 41, the sandwiching members can easily be detached from the ceramic honeycomb body 11 to which the coating material is applied.
However, the outer diameter of the dried outer peripheral walls 12 formed by the methods described in JP 5-269388 A and JP 2002-166404 A is smaller than the target outer diameter of the ceramic honeycomb structure 10, because the coating material shrinks due to the evaporation of water by drying. Accordingly, the ceramic honeycomb structure 10 is shaken in a metal container during use, resulting in the likelihood of breakage.
Particularly when a peripheral portion is removed from the ceramic honeycomb body 11 by machining as described in JP 2002-166404 A, the volume of notches should be increased so that broken peripheral portions and deformed cell walls 13, if any, can be removed. Accordingly, The outer diameter Db [see FIG. 4(a)] before forming the outer peripheral wall 12 may differ from one ceramic honeycomb body 11 to another. Further, when sintering is conducted after removing the peripheral portion from the ceramic honeycomb body 11 by machining, the outer diameter of the ceramic honeycomb body 11 differs more due to expansion and shrinkage in sintering.
Thus, when a coating material is applied to the outer peripheral surfaces 11a of ceramic honeycomb bodies 11 having different outer diameters Db, using support plates of a constant size as shown in FIG. 4(a), coatings 12c having different thicknesses Tc are formed on ceramic honeycomb bodies 11. Different thicknesses of the coatings 12c result in difference drying shrinkage degrees, failing to provide ceramic honeycomb structures 10 having constant outer diameters Dd after drying. Particularly when the outer diameter of the honeycomb structure 10 is smaller than the target size, the ceramic honeycomb structure 10 is shaken in a metal container during use, resulting in the likelihood of breakage.
In addition, a coating material cannot fully be applied to the outer peripheral surface 11a of the ceramic honeycomb body 11 near the end surfaces 15a, 15b by the methods of JP 5-269388 A and JP 2002-166404 A, gaps 17 are generated in boundaries 16 between the outer peripheral surface 11a of the dried ceramic honeycomb structure 10 and the resultant coating 12c as shown in FIG. 4(b), so that the outer peripheral wall 12 is easily cracked.